Macros NK_OFFSETOF, NK_CONTAINER_OF and NK_ALIGNOF rely on undefined behavior

When compiling with clang-9 with the option `-fsanitize=undefined` the basic nuklear example, I get the runtime errors :

`../code/nuklear.h:19196:30: runtime error: member access within null pointer of type 'union nk_page_data'`
`../code/nuklear.h:19197:34: runtime error: member access within null pointer of type 'struct nk_page_element'`

Indeed, the macros `NK_OFFSETOF`, `NK_CONTAINER_OF`, `NK_ALIGNOF` all dereference a null pointer.
One possibility might be to have a `NK_NO_UB` flag or something so that people who want to catch UB errors don't get them when using nuklear.

Start in nuklear.h around lines 19196-19197 and inspect the definitions and uses of NK_OFFSETOF, NK_CONTAINER_OF, and NK_ALIGNOF. Reproduce the report with the basic Nuklear example under clang-9 and -fsanitize=undefined; done means these macros no longer trigger the reported null-pointer member-access errors.

Written by the indexing model from the issue text.